During an extended listening session last night, I noticed a kind of screeching noise coming through the left channel at a lowish level. On closer inspection this morning, it seems that one of the power valves is making a mechanical noise which is being amplified to the speaker through microphony. To avoid amp damage if the valve blows, I have decided to take the amp out of action and have ordered some new valves. Unfortunately this means that I have no music until the new valves show up (which could be a while as I am ordering them from the states as they are miles cheaper from the US). I suppose in future I should always carry a spare set of valves to avoid the problem of being without music :(